1. Field of the Invention
The present invention relates to a stop control system for an internal combustion engine, which automatically stops and restarts the engine installed on a vehicle.
2. Description of the Related Art
Conventionally, as a stop control system for an internal combustion engine of this kind, one disclosed in Japanese Patent No. 4475437 is known. To improve fuel economy, the engine is automatically stopped during stoppage of a vehicle when predetermined stop conditions (idle stop conditions) are satisfied. Further, the engine is used as a drive source of a compressor of an air conditioner. This can cause the following inconveniences when the engine is stopped during a cooling operation of the air conditioner.
After stoppage of the engine, although the blowing of air into a vehicle compartment by an electric blower is continued, the circulation of refrigerant is stopped along with stoppage of a compressor, whereby the temperature of an evaporator starts to rise. Since air having passed through the evaporator is blown out into the vehicle compartment, the temperature of the air progressively rises as the temperature of the evaporator rises. This makes it impossible to maintain sufficient cooling performance of the air conditioner.
Therefore, in the stop control system disclosed in Japanese Patent No. 4475437, after the engine is automatically stopped, when a predetermined engine stoppage time period has elapsed, the engine is restarted. The engine stoppage time period is set in the following manner: First, an upper limit temperature of the evaporator of the air conditioner below which it is possible maintain the comfort of vehicle occupants in the vehicle compartment (hereafter referred to as the “allowable upper limit temperature”) is calculated according to the settings of the air conditioner and the external environment. Then, a time period over which the comfort of vehicle occupants in the vehicle compartment can be maintained (hereafter referred to as the “compartment temperature rise allowable time”) is calculated based on the difference between the calculated allowable upper limit temperature and the temperature of the evaporator detected immediately before the stoppage of the engine. Furthermore, the engine stoppage time period is set based on the calculated compartment temperature rise allowable time.
Engine coolant of the engine is used as a heat source of the air conditioner. When the engine is stopped during a heating operation of the air conditioner, the engine coolant of the engine starts to be lowered along therewith. This lowers the temperature of a heater, whereby the temperature of air blown out into the vehicle compartment via the heater is progressively lowered, thereby making it impossible to maintain sufficient heating performance of the air conditioner.
On the other hand, in the conventional stop control system described above, the engine stoppage time period for defining start timing of the restart of the engine is set in order to maintain the comfort of vehicle occupants in the vehicle compartment during a cooling operation of the air conditioner, but this setting of the engine stoppage time period is merely performed based on the temperature of the evaporator, irrespective of the temperature condition of the heater. Therefore, the engine is sometimes not restarted although the temperature of the heater is largely lowered. In such a case, the temperature of air blown out into the vehicle compartment cannot be maintained to be high, which results in the degraded comfort of vehicle occupants in the vehicle compartment provided by a heating operation of the air conditioner. Inversely, the engine is sometimes restarted quickly although the temperature of the heater is not so lowered, which degrades fuel economy of the engine.